baremetal: create an assert_fail C helper

Create a test for it under interactive/assert_fail.c

Move prompt under interactive

Create baremetal/arch/aarch64/c_from_as.S as an example of calling C
functions from assembly.

baremetal/arch/aarch64/c_from_as.S

@@ -0,0 +1,5 @@
+/* Call a C function. */
+.global main
+main:
+    mov x0, #0
+    bl exit

baremetal/interactive/assert_fail.c

@@ -0,0 +1,6 @@
+#include <common.h>
+
+void main(void) {
+	assert_fail();
+}
+

baremetal/lib/common.c

@@ -1,3 +1,4 @@
+#include <stdlib.h>
 #include <sys/stat.h>
 
 enum {
@@ -76,3 +77,8 @@ void _exit(int status) {
     );
 #endif
 }
+
+void assert_fail() {
+	puts("lkmc_test_fail");
+	exit(1);
+}

baremetal/lib/common.h

@@ -0,0 +1,4 @@
+#ifndef COMMON_H
+#define COMMON_H
+void assert_fail();
+#endif